vetkilr
I don't think there is a subwoofer on this market that can take 5000 watts true RMS. I'm pretty sure that would melt even 6 " voice coils very fast. 5000 watts is more power than 220 volt electrical outlet with 20amps. That is no joke. Hook up your subwoofer to your hot tub outlet and see how long it lasts at 60Hz. Sure you may be able to burp it for a second, but don't expect any kind of continuous output, power compression will set it very fast, and the thermal glue limit just after.
I strongly urge you to forget about 12" subwoofers. You should consider at least one 15" or better yet 18". You will have a lot more output without having to push more power into the subwoofer.
To give you an idea, and 18" with be about as loud as a 12" with the same motor with about 3 times the power!
That is assuming the motor is powerful enough to really drive the 18" effectively.
For example 4HP 12" vs. 4HP 18" There are many undermotor'ed 18's out there, don't expect any of them to work in a small box. The 18" 4HP can work in as little at 1.7cubic feet ported and as little as 0.57 cubic feet sealed for maximum depth extention in both cases.